It is often desirable to maintain the temperature of a reservoir of fluid, for example water, within a predetermined temperature range. Placing an immersion heater directly in the reservoir will, of course, heat the water, but it is likely to introduce thermal gradients and convection currents which will cause the temperature in the reservoir to vary from one point to another. Thus, where it is important to maintain the temperature of the entire reservoir uniform without thermal current, a closed circulating system can be used into which an immersion heater may be placed at a point removed from the reservoir.
One particular system that requires a reservoir of uniform temperature is an ultrasound mammary scanning apparatus of the kind disclosed in U.S. Pat. No. 4,282,880, issued Aug. 11, 1981. The inventors listed on U.S. Pat. No. 4,282,880 are the same as the inventors of the present invention, and that patent is assigned to the assignee of the present patent application. The specification of U.S. Pat. No. 4,282,880 is incorporated herein by reference.
In the system described in this patent, a scanning transducer and associated sonic less are submerged in water. Subjects are positioned on an examination table with the breast projecting through an aperture in the table into the tank of water in which the ultrasound equipment is enclosed. It is necessary to condition the water used for the ultrasound transmission medium to remove sources of interference that could distort the ultrasound image. Water conditioning may involve filtering of particulate matter, removing bubbles, inhibiting bacteria growth and maintaining uniform temperature to eliminate thermal gradients, all of which can interfere with ultrasound transmission. In the water circulation and maintenance system disclosed in this U.S. Pat. No. 4,282,880, the water transmission medium is divided into two separate pools by means of a flexible bag. The first pool consists of the water collected in the main tank in which the ultrasound transducing equipment is placed. The patient's breast is suspended in a separate but smaller pool of water separated from the water in the main tank by a flexible bag. Thus, the portion of the water in contact with the patient may be changed regularly for aesthetic and sanitary reasons without having to change the water in the entire system.
When the water in the bag is changed, it is necessary to filter particulate matter, remove bacteria and equalize the temperature of the newly added water to that of the water which remains in the main tank.
The temperature of the water in both pools is maintained uniform by circulating in water through two separate but interdependent circulating systems, both of which include an immersion heater. The immersion heater may be spliced into the fluid circulation line using a conventional T-joint, with one arm of the T-joint providing an inlet, a second arm of the T-joint providing an outlet and the third arm of the T-joint supporting the heater itself. The space provided inside the T-joint acts as a reservoir to permit water to circulate completely about the heating element.
In systems such as that described in U.S. Pat. No. 4,282,880, the amount of water maintained in the system and the circulation flow rates are both low, so that it is not practical to use large immersion heaters. What is required is a relatively small immersion heater.
We have found that when such heaters are spliced into a circulating system of this kind, air bubbles can be generated around the base of the heater and can be trapped so that a large air bubble can build up which permits the heater to overheat and sometimes burn out. It is confined system such as this, where it is not feasible to provide large, separate reservoirs for the heating element, it is necessary to find a method for preventing this bubble build-up around the base of the heater.